The invention concerns a process and apparatus for the purification of raw gasoline from catalytic cracking.
The production of reformulated gasoline satisfying new environmental regulations requires, in particular, a reduction in the concentration of olefins and/or aromatics (especially benzene), also sulphur, and particularly mercaptans.
As an example, the presence of diolefins in catalytic cracking gasolines risks the formation of gums which mean that such raw gasolines are difficult to use as a fuel.
The diolefins must therefore be eliminated before etherification.
We have already developed a process for selective hydrogenation of a catalytic cracking gasoline which eliminates diolefins and which consists of bringing the feed into contact with a catalyst containing 0.1-1% of palladium deposited on a support. Such a process is described in European patent EP-A-0 685 552.
Further, oxidizing sweetening is a reaction which is well suited to ensuring that malodorous compounds in catalytic cracking gasolines do not pass into the gasoline pool.
A sweetening process has been described in EP-A-0 638 628 which consists of bringing the cut to be treated into contact, in the presence of air, with a catalyst comprising an alkaline aluminosilicate, activated charcoal and a metal chelate.
Unfortunately, when gasolines which contain a large quantity of mercaptans (at least 120 ppm) are treated, in order to obtain a mercaptan level which satisfies the regulations, low space velocities or large quantities of catalyst must be used, or a plurality of sweetening reactors must be used. These constraints are highly problematic for the operator.
We have, therefore, developed a process which can overcome these disadvantages and which also improves the service life of the sweetening catalyst.
More precisely, in the process of the invention the feed (catalytic cracking gasoline) containing dienic and/or acetylenic impurities and mercaptans, undergoes selective hydrogenation, the effluent obtained is stabilized then undergoes sweetening, and the gasoline obtained is degassed.
The process of the present invention has a number of advantages:
reduction of the diolefin concentration to less than 3000 ppm, preferably 2500 ppm and more preferably 1500 ppm;
displacement of the double bond in some branched olefins, for example 4-methylpentene-1 to 2-methylpentene-2, thus increasing the quantity of etherifiable olefins;
sweetening by a catalytic reaction between rercaptans and diolefins leading to the formation of sulphides, or by an oxidising catalytic reaction to convert mercaptans to disulphides, the sulphides and disulphides being readily eliminated;
when the selective hydrogenation step is operated at a temperature of 80xc2x0 C. or more and the sweetening step is preferably carried out at 80xc2x0 C. or less, there is good thermal integration in the process,
the selective hydrogenation temperature is controlled by recycling a portion of the sweetening effluent (dedienized, sweetened and cooled gasoline) to the selective hydrogenation step.
The invention also concerns an apparatus for carrying out the process of the invention for the purification of catalytic cracking gasolines containing dienic and/or acetylenic impurities, and mercaptans, said apparatus comprising at least one selective hydrogenation reactor containing at least one fixed catalyst bed, and having at least one line for introducing a feed, at least one effluent outlet line, and a line supplying hydrogen to the reactor, said reactor being followed by at least one stabilization drum connected to said effluent outlet line, the drum having at least one gas outlet line and at least one stabilized effluent outlet line, and said effluent passing into at least one sweetening reactor comprising at least one effluent inlet line and at least one effluent outlet line, said reactor having close thereto at least one oxidizing agent supply line, said apparatus also comprising at least one drum for degassing the effluent from the sweetening reactor, said drum having at least one gas outlet line and at least one outlet line for dedienized, stabilized and sweetened gasoline.
This integrated process can also reduce the investment required compared with conventional processes, since:
the two reactors can be operated without the need for additional pumps, with the exception of the recycling pump when necessary;
the reduction in the mercaptan content as early as in the selective hydrogenation reactor can considerably reduce size of the sweetening reactor.